Among other concerns, opponents of Keystone XL say the pipeline would pass through the Sandhills, an ecologically fragile region in Nebraska of grass-covered sand dunes, and would cross the land of farmers and ranchers who don't want it.

The commission, however, is specifically prohibited from evaluating safety considerations, including risk or impact of a spill, and will instead rule on issues including regulatory compliance, economic and social impacts of the project, the potential intrusion on natural resources, and whether better routes exist.

Barack Obama rejected Keystone XL in 2015 after years of review, only for President Donald Trump to give the go-ahead to the project earlier this year.